Physicochemical characterization of combustion generated inorganic nanoparticles

Abstract

Combustion generated particulate matter is widely recognized to be the major pollutant in urban areas because of stationary combustor and vehicular traffic sources. Particularly the ultrafine fraction was proven to have the highest impact on human health since its ability to deeply penetrate the lung and the circulatory system. It is also the most reactive fraction, enriched in toxic condensable species, because of the large specific surface area. Up to recent times the nanometric fraction, i.e. below 10nm, was not taken in consideration, despite it is the most toxic, because it was assumed that such small particles meet and coagulate at gas kinetic rate so that their lifetime was negligible small. Some recent results on carbonaceous nanometric particles generated in laboratory flames showed that, at high temperatures, their coagulation as well their collection efficiencies drops dramatically orders of magnitude as their size decreases below 5nm. Consequently they could escape from combustors and filters and survive in the atmosphere in a not negligible amount. It is well know that metals, present as ashes or additives into practical fuels, brought at high temperature in combustion devices, generate a large range of particulate which extend in the ultrafine range. The fraction with size smaller than 10nm of such inorganic particles was not yet studied. The measurements routinely performed at the exhaust of combustors does not extend in this size range despite it is well known that the addition of trace amount of metals into a flame causes nanoparticles formation. The objective of the research described in this thesis is to characterize such small toxic metal nanoparticles and to verify if they could be released at the exhaust of combustion systems because of coagulation and collection efficiencies following, at high temperature, the same trend of carbonaceous nanoparticles. The thesis is composed of three main parts, roughly corresponding to the three year of the re-search activities. The first two chapters contains the background information on combustion generated particles and aerosol dynamic. The chapters two and three describe the experimental methodologies while the third parts contain the obtained results and their discussion. Reactors consisting of flat laminar premixed flames doped with real fuels or toxic metal par-ticles precursors were developed and successively investigated. They allow performing spatially re-solved measurements, at several residence times in flame, of incipient inorganic nanoparticles generated in a well controlled environment. Diagnostics with high sensitivity to particles of few nanometers in size were chosen to this aim. They are on-line sampling probe high resolution Differential Mobility Analysis and in- situ Laser Light Scattering. Particles thermophoretic collection for extra-situ Atomic Force Microscopy Analysis was widely used too. Three main results have been obtained. Toxic metal nanoparticles behave quite similar to carbonaceous nanoparticles. Both their coagulation and collection efficiencies decrease by orders of magnitude with decreasing sizes below 5nm. The values of those efficiencies quantitatively agree between the three different gen-erated toxic metal nanoparticles and with that of carbonaceous nanoparticles. This imply that the physic behind the nanoparticles long lifetime and rebound on surfaces is stronger influenced by size more than by chemistry. Flames doped with pulverized coal contain a prevalent number of particles with size lower than 10nm whose contribution to volume fraction is not negligible too. Thus combustors burning ash containing fuels may be a source of such small nanoparticles whose emission involves a not negligible pollution problem.